The object of these investigations is to elucidate the mechanisms responsible for the phenomenon of myocardial reactive hyperemia in the dog and to examine the possibility that reactive hyperemia in the human heart at the time of revascularization surgery can serve as an indicator of vasodilator reserve as well as the adequacy of the revascularization procedure. These studies will be carried out in a canine model and in the human heart at the time of coronary artery surgery. Coronary blood flow will be measured using electromagnetic flow transducers. Reactive hyperemia will be produced by temporary coronary artery or saphenous vein occlusion. In the dog, physiological, pharmacological and histochemical studies will be carried out while in man, phasic flow and reactive hyperemia will be recorded and correlated with angiographic, anatomic findings and long term patency rates.